Certain applications in the generation, transmission, distribution, and utilization of electrical energy require the reversal of phases of a polyphase electrical system. For example, pumped-storage electrical generation projects utilize a dual mode electric machine as a pump motor to pump water into a reservoir to increase the head behind a dam. This pumping occurs during off-peak hours. During peak demand for electricity on a power grid, the reservoir is drained to drive the electric machine as a generator to produce electric power for the grid. The electric machine is switched between pump motor and generator operation by reversing the phase connections to the machine.
To provide phase reversal in a three-phase electrical system, it is known to use a five-pole non-segregated reversing switch and a mechanism for operating the poles in the proper sequence to prevent short circuiting. In operation, three poles are closed at the same time that two other poles are opened in a disconnect position. To reverse polarity, the two other poles are closed when two of the other three poles are opened. In this manner two of the three phases are reversed. This type of switch is relatively expensive to manufacture due to the use of five poles. Further the five poles usually require a significant amount of room within a switch gear enclosure.